mapping ocean depths - clark science center

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Take away the water and what do you see?

Physiography of the Ocean Floor

Earth’s dominant topographic features lie beneath the ocean’s surface.

Distribution of topography and bathymetry

Bimodal distribution

Average land elevation 0.84 km

Average ocean depth 3.8 km

Deepest depth (Mariana Trench) more extreme than highest height (Mt. Everest)

Ocean provinces

The submarine “landscape,” or sea floor, can be subdivided into three distinct provinces:

continental margins~22% of total area

deep ocean basins~42% of total area

mid-ocean ridges~31% of total area

(remaining ~5% of total area occupied by deep-sea trenches)

Continental margins

submerged edges of the continents massive wedges of sediment eroded from the land and deposited along continental edgewide, flat in Atlanticthin, steep in Pacificcan be divided into three parts:

continental shelf – continuation of land below sea surface (slope < 1o)continental slope – where margin breaks (slope ~ 4o); often contain huge submarine canyons where sediment cascades downcontinental rise – sedimentary wedge at base of slope (slope 1o or less)

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Continental margins

Grand Canyon

- 6,672 ft.

Monterey Canyon

Submarine canyon off Barcelona, Spain

submarine canyons

Deep ocean basins

between the continental margins and the mid-oceanic ridge

includes a variety of features from mountainous to flat plains:

abyssal plains – flattest parts of the worldabyssal hills – elongated dome-shaped hills of oceanic crustseamounts – abyssal mountains, largely volcanic (active and extinct); includes flat-topped guyots formed by wave erosion deep sea trenches – deepest regions on Earth, found close to land

Mid-ocean ridges

continuous submarine mountain rangeextends for about 60,000 km around the Earthcontains rift valley in center of ridgesite of new oceanic crust formationflanked by transform faults and fracture zones

Prior to the early 20th century, “soundings” were the only means to determine water depth

weighted lines lowered from ships

time-consuming, relatively few, doubtful accuracy

HMS Challenger one of the first to do this

Mapping ocean depths

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Matthew Maury(the “father of oceanography”)constructed the first ocean-wide bathymetric map(N. Atlantic, 1855).

Bathymetric maps

depict the topography of the seafloor;

isobathsconnect points of equal elevation.

Echo Sounder

Seismic ReflectionProfiler (sound sourceand hydrophone)



echo sounders

sound source and receiver (hydrophone) on hull of shiphigh frequency sound waves travel through the water, reflectoff the seafloor (and off sub-bottom sediment layers), and are recordedby the hydrophone based on travel time (t) of sound waves with known velocity (v), get depth [d = v x (t/2)]provide continuous depth profilesalong a ship’s cruise trackbut only 2-dimensional profile (horizontal, vertical)

sound source and hydrophone towed by ship

continuous but also only 2-dimensional profile

lower frequencyenergy source (stronger sound source with fewer sound waves per second)

deeper penetration into sedimentary layers and ocean crust

seismic reflection profilers

vs.


seismicreflection survey

guyotseamount

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like echo sounder, but images a 60 km swath of seaflooroverlapping swaths = complete coverage (3-dimensional)produces 3-dimensional bathymetric mapof seafloor


Multi-beam sonarand

Side scan sonar


Seabeam Multibeam bathymetry

Sea ice keel scour

bathymetric map based on aseabeam surveyfor Deep Sea Drilling Project sites off northwest Africa(contour interval = 50 m); note the very steep Mazagan Escarpmentdropping off to the deep-sea

isobaths


Precise altimeters (using microwaves) can map the relief of the ocean surfacewith centimeter-scale resolution.

Altimetry uses satellites to determine bathymetry based upon slight changes in the elevation of the sea surface

Gravitational attraction of large rock masses on the seafloor distorts the ocean surface, very slightly but enough to be measured from space!


Satellites

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North Atlantic Ocean relief from

satellite altimetry

mapping ocean depths - clark science center

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